1. Field of the Invention
This invention relates generally to lifting and leveling subsea structures, and more particularly to an apparatus for latching onto a subsea structure, lifting and leveling the structure, and positively unlatching from the structure without a dedicated signal being sent to the latching mechanism.
2. Brief Description of the Prior Art
In the drilling and production of wells, such as oil and gas wells beneath bodies of water, particularly at offshore locations, it has become accepted practice to install structures on the floor of the body of water known as "templates." These templates serve to provide a base, or support, for the landing and connection of various drilling and production equipment. Templates also provide for the connection thereto of anchors for towers and gravity anchors, or mooring devices. Templates can be anchored to the floor of the body of water by grouting the structure to anchoring piles which have been driven downwardly into the earth below the floor of the body of water. Templates can also be anchored by swaging the downwardly driven piles to the template structure, thereby forming a mechanical attachment of the downwardly driven piles to the template structure.
Often, the ocean floor is not level at the point of installation of the templates. The templates must either be allowed to repose at the angle of the ocean floor or be held in place at a desired plane by known devices. It is desirable to elevate the template to a horizontal plane to facilitate the landing of various drilling and production equipment used in the conduct of drilling, production or workover operations.
Previous methods of bringing a deployed template to a horizontal plane have, so far as is known, utilized either a mud mat-based elevating system or a pile-based leveling system. In a mud mat-based system, the equipment used to elevate the template to a horizontal plane was deployed with, and was an integral part of, the template structure. Inasmuch as the exact extent of required leveling was not known prior to deployment of the template, leveling devices were normally installed integral to the template structure at all corners of the template, even though they later turned out to be unnecessary.
In a pile-based leveling system, the structures used to raise the template to a horizontal plane were separate from the template structure and were deployed as a self-contained structure after the template had been landed on the ocean floor and foundation piling had been driven downwardly through the template structure. Pile-based leveling devices attached themselves to the template structure and reacted against the downwardly driven piles to achieve the leveling forces required to elevate the template to the horizontal plane.
Methods of latching pile-based leveling structures to the template structure have been primarily limited in the past to hydraulically actuated dogs which extended radially to mate with corresponding load bearing surfaces incorporated into the template structure. Other pile-based leveling devices incorporated sets of opposing wedges to grip onto the template structure. These wedges relied on the mass of the template to maintain their grip on the template structure.
A major drawback to the hydraulically actuated dog engagement, as well as the opposing wedge engagement, is that both required an operator or person remotely located on a vessel from the leveling device to perform a sole or dedicated function, that of enabling the connection of the leveling structure to the template. To perform this connection, a command signal was required to control the remote connection operator, requiring an additional control line, when the signal was hydraulic or electrical, between the vessel and the leveling structure. Acoustical control signals were often difficult to detect.
Another major drawback with the previous pile-based leveling structure was that of releasing the leveling structure once the template had been elevated, leveled and locked in position. In the event of a malfunction in the release mechanism system, retrieval of the leveling structure might not be possible. In most pile-based leveling operations, a single leveling device was used. Should the leveling structure not release after leveling of one portion of the template, no further leveling could be accomplished until the latch mechanism was released or unless backup lifting devices were available.
The present invention is distinguished over the prior art in general by a leveling apparatus which is lowered by rigging onto a structure, such as a subsea template, having one or more latching receptacles secured thereon and through which a pile has been driven into the ocean floor. The apparatus engages the open end of the pile and releasably latches onto the receptacle. The apparatus has a lift frame at its upper end, a landing ring below the lift frame, a cylinder frame below the landing ring, and a latch frame below the cylinder frame. The lift frame is connected to the latch frame by vertical stabilizer columns secured to the lift frame and connected at their lower ends to latches in the latch frame. The landing ring is slidably mounted on the stabilizer columns and hydraulic lift cylinders secured to the cylinder frame have their rod ends secured to the housing of the latch frame. When the apparatus is suspended by rigging, the landing ring and lift frame are spaced vertically apart and after the landing ring has been engaged on the pile, the rigging goes slack, and the upper lift frame moves vertically downward. The hydraulic cylinders are extended until the latches travel past a load shoulder in the receptacle and are then retracted to positively lock the latches on the receptacle load shoulder. Continued retraction raises the receptacle and connected structure to a desired position so the structure may be secured to the pile. The apparatus is removed by extending the cylinder rods to raise the lift frame and release the latches or by tugging on the lift frame with the rigging.